Stereophonic receiving apparatus



July 3 1963 J. w. PERCIVAL ETAL 3,099,714

STEREOPHONIC RECEIVING APPARATUS 3 Sheets-Sheet 2 Filed March 11, 1960 INVENTORS Joseph W. Percival 8 Rlchord W Cook I ATTORN EY WITNESSES f fiy July- 30, 1963 J, w. PERCIVAL ETAL 3,099,714

STEREOPHONIC RECEIVING APPARATUS 5 Sheets-Sheet 3 Filed March 11, 1960 155i? w 2222 2; y

I F Currier Frequency United States Patent 3,099,714 STEREOPHONIC RECEIVING APPARATUS Joseph W. Percival, Colonia, N.J., and Richard W. Cook,

Michigan City, Ind., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 11, 1960, Ser. No. 14,395 2 Claims. (Cl. 179-15) The present invention relates to apparatus for reception of single carrier wave Stereophonic signals and more particularly to apparatus for receiving signals in which a compatible monophonic component is supplied as amplitude modulation and Stereophonic intelligence is supplied as angle modulation of the same carrier wave.

When sound is transmitted by the ordinary system over radio channels, only a single channel is normally provided and audio perspective is entirely lost since the phase and amplitude differences between the sounds received by the two cars of the listener have lost the spatial displacement information of the sound sources which feed the transmitter. Stereophonic reception has heretofore been demonstrated using two microphones, set up at locations on each side of a stage on which an orchestra is situated. Each microphone is connected by a separate radio channel to one of two loudspeakers placed similarly as the microphones but in a listening chamber. By such an arrangement an auditory effect may be obtained which is substantially the same as though the orchestra or other source of sound were actually located in front of the listener rather than the sound being reproduced by the loudspeakers.

In applying stereophonic sound to radio broadcasting the need for two separate channels for the transmission of a single program has prohibited stereo transmission in the AM broadcast band. Commercially successful stereo broadcasting awaits development of compatible apparatus and methods which eliminate the need of two separate radio channels. Solutions to the problem must permit transmission of both audio signals on the same carrier frequency, thereby using but one radio channel and re ducing to a minimum the additional investment required at the transmitter as well as the additional investment required of prospective listeners.

One proposed stereophonic system using a single channel has been described in detail in Electronics magazine, issue of February 1941, on pages 34 to 36. That proposed system suggests transmission of the audio signal from a first microphone as amplitude modulation and that from a second microphone as frequency modulation of the same carrier. Such a system has the disadvantage that a conventional receiver will reproduce the signals from the first microphone only. Thus a conventional receiver produces sound corresponding to that heard at one end of the orchestra. A primary requisite of a genuinely compatible system is that a conventional AM broadcast receiver should produce balanced monophonic sound substantially corresponding to the sound which would be heard by a listener seated near the center of the auditorium in which the orchestra is located.

One Stereophonic broadcasting system which overcomes the foregoing problems and permits compatible monophonic reception with conventional AM receivers is de- "ice scribed in detail in copending application of Harold E. Sweeney and Charles W. Baugh, Jr., Serial No. 808,038, filed April 22, 1959, and assigned to the same assignee as the present application. In that system amplitude modulation of a broadcast band carrier wave is used for transmission of the algebraic sum A+B of the two stereo signals and frequency modulation of the same carrier wave is used for transmission of the difference signal AB. The transmission system there utilized enables a conventional AM receiver tuned to the transmitted carrier to reproduce the sum signal A+B which signal contains substantially equal components of the signal from each microphone and therefore will produce a balanced monophonic sound. The aforementioned copending application proposes a special receiver for use with the transmission system which receiver would have two separate detectors one being an amplitude modulation detector and the other being an FM detector so that the A+B audio signal and the AB audio signal are separately produced at the outputs of the two detectors. The two outputs in such a system are then matrixed by known sum producer and diference producer networks to provide separate audio signals A and B which are coupled respectively to the separate loudspeakers.

In copending application Serial No. 805,992, filed April 13, 1959, by H. E. Sweeney and assigned to the same assignee as the present application there is described a receiver for signals of the abovementioned type in which the A+B amplitude modulation signals and the AB frequency modulation signals are both received by each of a pair of amplitude modulation receiving channels which channels are adjusted differently relative to the received carrier center frequency so that one channel will produce the first audio signal A and the other channel will produce the second audio signal B. More specifically, the first receiving channel is tuned so that the received carrier, after heterodyne translation to an intermediate frequency, is located on the positive slope of the receiver frequency response characteristic. The other receiving channel is tuned so that the received carrier signal lies an equal dis tance down on the negative slope of the frequency response characteristic of the second receiver. Each receiver channel will detect and reproduce the monophonic component A+B. In addition the first receiver channel will slope detect the frequency modulation signal to produce a Stereophonic difference signal component A-B and will add the sum signal A-l-B and the difference signal AB to produce a signal corresponding to the sound signal A from the first microphone or sound source. Similarly the second receiving channel will slope detect the frequency modulation signal and will algebraically subtract the demodulated difference signal AB from the demodulated sum signal A+B to produce a coherent sound signal corresponding to the signal B of the second microphone or sound source.

Tlaus, when two generally conventional AM receivers are tuned to a carrier wave of the type disclosed by the abovementioned application 808,03 8, with one being tuned to a frequency slightly lower than the carrier center frequency and the other being tuned slightly higher than the carrier frequency, s-tereophonic reproduction of the transmitted sound may be accomplished. Stereo reception is possible because the detuning allows slope detection of the FM (AB) signal. Slope detection by a set tuned low will result in, say, an AB signal, being detected, also the A+B signal will be AM detected. These signals will combine to produce 2A, one of the original stereo signals. A similar result will be obtained when the other receiver is tuned high with the difference that in this case FM detection takes place on the opposite slope of the passband thus resulting in a phase inversion of the A-B signal to give BA. The A-l-B signal is again AM detected and combines with B-A and results in an output of 2B.

Another stereop honic broadcasting system which perrnits compatible monophonic reception with conventional AM receivers is described in Electronics magazine, volume 33, No. 3, January 15, 1960, on page 45. In that system there is transmitted a single radio frequency carrier wave having two independent sidebands which are respectively modulated by the two stereo informations A and B. Compatible monophonic reception is had by tuning a conventional AM broadcast receiver to the carrier center frequency in the usual manner so' that equal portions of the two sideband signals are detected to produce A+B. Stereophonic reception may be achieved by positioning two AM receivers about five to nine feet apart and side tuning the receivers respectively to the upper and lower sidebands. The first rereiver detects the lower sideband while substantially rejecting the upper, and, vice versa, the second receiver detects predominantly the upper sideband signal.

It will be apparent that the two different aforesaid stereophonic broadcasting systems are similar in that both enable stereophonic reception by opposite side tuning of a pair of AM receivers.

In theory the foregoing side tuning" technique will produce perfectly balanced stereo signals A and B at the two spaced loudspeakers, however, in practice, considerable skill is required to equally detune the two channels just enough to completely eliminate the A component in one channel and the B component in the other. The tuning operations are particularly difficult because maximum gain and correct side tuning do not coincide. Since the person at the receiver has no knowledge of What portion of the transmitted program is emanating from the left side of the orchestra and what is from the right side, the only satisfactory tuning method is to put one hand on each tuning control and vary them simultaneously to achieve a maximum stereo effect.

Usually it will be desirable to space the two receivers some five to ten fee-t apart in a listening space. Under such circustances it is virtually impossible to simultaneously adjust both tuning controls. Moreover, when the person is located within reach of the tuning controls he is not positioned in the optimum area for stereophonic listening *and will experience difficulty in balancing the tuning of the receivers.

It is a primary object of the present invention to enable stereophonic reception by combined utilization of two AM broadcast receivers tuned to the same broadcast band carrier Wave and to provide such receivers with means to facilitate individual tuning for separate detection and reproduction of the left hand and right hand stereo signals.

It is another object of the present invention to provide stereophonic radio receiving apparatus including a pair of spaced loudspeakers which apparatus may be tuned for production of balanced monophonic sound in concert from both loudspeakers and alternatively may quickly and accurately be adjusted for balanced stereophonic sound production.

It is a further object of this invention to provide a pair of radio wave receivers with continuously variable tuning means whereby each may be tuned for maximum response to a carrier wave and to further provide each with a discontinuous or step-type tuning means for thereafter altering the tuned frequency of each receiver by a predeter- '4; mined frequency differential to place the carrier wave at corresponding positions on opposite slopes of the frequency response curves of the two receivers.

It is another and more specific object of the present invention to provide a single receiver having a pair of like intermediate frequency amplifiers the input circuits of which are coupled to a common heterodyne converter and to provide switch means associated with the resonant circuits of each of said amplifiers for selectively increasing the resonant frequency of one and decreasing that of the other to thereby dispose the intermediate frequency carrier signal on a positively sloping frequency response portion of the one and on a negatively sloping portion of the frequency response characteristic of the other.

The foregoing and other objects and features of the present invention will be more clearly apparent from the following description taken with the accompanying drawing, throughout which like reference characters indicate like parts, Which drawing forms a part of this application and in which:

FIGURE 1 is a block schematic diagram showing a stereophonic receiving system in accordance with the present invention;

FIG. 2 is a block schematic diagram of a second embodiment of a receiving system in accordance with the present invention; and

FIG. 3 is a plurality of voltage waveform curves and frequency response curves useful in achieving full com prehension of the invention.

One transmitting apparatus for providing a multiplex signal of a type usable by the receivers of the present invention is described in detail in the aforementioned copending application Serial No. 808,038. Accordingly, it will suffice for present purposes to state that such a trans rnitter system provides a composite signal including a carrier wave which is amplitude modulated and frequency modulated respectively with a sum signal A+B and a difference signal A-B wherein A and B are coherent audio signals respectively corresponding to sound intelligence at two spaced locations within range of a sound source such as an orchestra.

In FIG. 1 there is shown one embodiment of an apparatus for receiving signals of the type produced by a transmission system such as that described above. FIG. 1 shows a pair of radio receivers for receiving signals in the AM broadcast band. The receivers are conventional in many respects and particularly in that they respectively include receiving antennas 30 and 40, conventional heterodyne converter circuits 32 and 42, local oscillators 34 and 44, conventional AM detector circuits 3'7 and 47, conventional audio amplifiers 38 and 48, and conventional loudspeakers 39 and 49. In accordance with the present invention, the loudspeakers 39 land 49" are spaced apart a predetermined distance, preferably of the order of five to nine feet, in a listening chamber. The first receiver of FIG. 1 includes an intermediate frequency amplifier and bandpass filter 36 having a frequency response characteristic substantially as shown by curve 35 in FIG. 1. Similarly, the second receiver includes an intermediate frequency amplifier and bandpass filter 46 having a frequency response characteristic substantially as shown by curve 45.

In accordance with the present invention oscillator 34 has a resonant tank circuit comprising inductor 11 shunted by series connected capacitors 12 and 13. Capacitor 12 preferably is a conventional rotary tuning capacitor which is variable from about 20 to micromicrofarads by means of a conventional continuous tuning control 31. A similar variable tuning element of conventional form (not shown) in the radio frequency selective circuit of converter 32 is ganged with capacitor 12 and tuning control '31. The tank circuit further includes step-tuning means comprising a single pole three position switch device having its movable arm 15 connected to the junction of capacitors 12 and 13. Stationary contact 16 of the switch is connected through a fixed capacitor 14 to a point of reference potential shown as ground. Contact 18, one end of inductor 11 and one end of capacitor 13 are all connected to the same point of reference potential. Contact 17 is unconnected to the tank circuit. When switch arm 15 is positioned to contact terminal 18 capacitor 13 is shorted out and the capacitance shunted across inductor 11 has a maximum value corresponding to the adjusted capacitor 12. Accordingly, the resonant frequency of the tank circuit is at its minimum value for that particular setting of capacitor 12. When switch arm 15 is moved to terminal '16, capacitor 14 is shunted across capacitor 13 and the parallel combination is connected, in series with variable capacitor 12, across inductor 11. Accordingly, the tank capacitance is reduced by a fixed predetermined amount and the resonant frequency is increased in a single step by an amount dependent upon the values of capacitors 13 and 14. When arm 15 is moved from terminal 16 to terminal 17, capacitor 14 is disconnected from across capacitor 13, thereby further reducing the tank capacitance by a similar predetermined amount and further increasing the resonant frequency of the oscillatory tank circuit. Capacitors 13 and 14 preferably are of substantially equal value (about .005 microfarad in one embodiment) so that switching from contact 15 to contact 17 increases the oscillator frequency a predetermined number of kilocycles, while switching from contact 16 to contact 18 decreases the oscillator frequency by a like amount.

The tank circuit of the second receiver of FIG. 1 is similar to that of the first receiver with the elements 2.1 through 28 corresponding respectively to components 11 to 18. Accordingly, the connections of components 21 through 2-8 are not described in detail.

In utilizing the receivers of FIG. 1 in accordance with the present invention the two receivers are respectively tuned to the same AM broadcast carrier by means of conventional tuning controls 31 and 41. When so tuned the receivers produce balanced monophonic sound in concert. That is true regardless of whether the carrier wave is modulated in accordance with the aforementioned copending application Serial No. 808,038, or in accordance with the Electronics article stereo system, or by an ordinary monophonic AM broadcast signal.

To achieve stereo reception of either of the aforedescribed stereo broadcast signals the switch 15- is moved from contact 16 to the right contact 17 and switch arm 25 is moved from contact 26 to the left contact 28. The frequency of oscillator 34- is thereby increased in a single step by a predetermined amount, and the frequency of oscillator 44 is correspondingly decreased.

Mixer 32 accordingly produces a modulated intermediate frequency carrier signal having a predetermined center frequency f which lies midway on the positively sloping portion of the frequency response characteristic curve 35. The intermediate frequency carrier signal will of course have the same frequency modulation and amplitude modulation as that of the incoming signal as received by antenna '30. Assuming that the .received carrier is of the type broadcast by the system of copending application Serial No. 808,038, the intermediate frequency carrier will vary in frequency on both sides of the center frequency f between the deviation limits f and f as shown on curve '35. Also the IF carrier will vary in amplitude in accordance with the sum signal A-l-B. When the so modulated intermediate frequency carrier is applied to the sloping portion of the bandpass curve 35, the IF amplifier 36 will amplify signals at frequency f to a greater degree signals at frequency f Accordingly, lF amplifier 36 operates to convent the frequency modulation to an amplitude modulation component. That is, in response to the A-B frequency modulation, the IF amplifier 36 generates an amplitude modulation component which .varies in direct proportion to the instantaneous frequency of the interme- 6 diate frequency carrier as determined by the frequency modulation thereof.

The so generated amplitude modulation component, which corresponds to the stereophonic difference signal AB, is additively combined with the pre-existing amplitude modulation of the intermediate frequency carrier, which pre-existing amplitude modulation corresponds to the stereophonic sum signal A -l-B. Accordingly, the amplitude modulation of the intermediate frequency carrier at the output of amplifier 36 has an envelope which varies substantially in accordance with the algebraic sum of the sum signal A -l-B and the difference signal A-B. The algebraic sum corresponds to the signal A and is substantially representative of the sound intelligence impinging upon the first microphone A of the transmission system as shown in FIG. 1. The IF signal from amplifier 36 is rectified and filtered by detector 37 in the conventional manner and the resultant audio signal A is amplified by audio amplifier 38 and applied to the first loudspeaker 39.

When the first receiver 3039 is adjusted, as above, to produce the right hand stereo signal A, the second receiver 40-49 preferably is adjusted to reproduce the left hand signal B. Such adjustment comprises positioning switch arm 25 to terminal 28 thereby shorting out capacitor 23 and decreasing the resonant frequency of oscillator 44. Accordingly, local oscillator 44 of the second receiver is tuned higher than normal in frequency so that it beats with the incoming radiofrequency carrier as received by the antenna 40 to produce an intermediate frequency carrier wave having a center frequency f which is higher in frequency than the maximum response portion of the IF frequency response characteristic as shown by the curve 45.

The intermediate frequency carrier wave as applied to the IF amplifier 46 will have a center frequency f as shown in conjunction with the curve 45, and will vary in frequency between the limits f and 12;, with the variations in frequency corresponding to the frequency modulation of the incoming carrier. When the stereophonic difference signal AB has a maximum positive value the intermedi ate frequency carrier will approach the limit frequency f Likewise, when the stereophonic difference signal AB has a maximum negative value the carrier will approach the limit frequency f;;. When the sound signals received by the microphones A and B are identical both in amplitude and in phase, the difference signal A-B will have a zero value. In that instance the frequency of the intermediate frequency carrier as applied to IF amplifier 46 will have a frequency corresponding to the center frequency f When the stereophonic dilference signal has a negative value, the modulated carrier will approach the frequency i and the IF amplifier 46 will have a maximum amplification factor. When the stereophonic difference signal has a maximum positive value, the carrier will be located at f and IF amplifier 46 will have a minimum amplifiication factor. Accordingly, the IF amplifier 46 generates an amplitude modulation component which varies in invense proportion to the instantaneous frequency of the intermediate frequency carrier and, therefore, in inverse proportion to the amplitude value of the stereophonic difference signal A-B. Thus, the amplitude modulation component generated by the amplifier 46 responding to the frequency modulation has the form (A-B). That inverse function amplitude modulation component is combined with the pre-existing amplitude modulation of the IF carrier as received by the antenna 40. Accordingly, the amplitude envelope of the output signal from 'IF amplifier 46 varies in accordance with the :alegbnaic difference of the sum signal (A +B) and the estereophonic difference signal (A B), and is substantially representative of the audio signal B applied to the left hand microphone at the transmitting studio. The composite amplitude modulation of the intermediate frequency carrier is detected by the conventional AM detect-or 47. The audio signal B thusproduced is applied through the conventional audio amplifier 48 to the second loudspeaker 49.

In FIG. 2 there is shown a second embodiment of receiving apparatus employing the concepts of the present invention. The antenna 50, heterodyne converter 51 and the intermediate frequency amplifier 52 preferably are conventional circuits such as commonly used in AM broadcast receivers. The output of intermediate frequency amplifier 52 is coupled to la fir'st channel cornprising an intermediate frequency bandpass filter 86 which is coupled to a conventional amplitude modulation detector 87. The output terminals of the AM detector 87 are connected to a conventional audio amplifier 88 which applies signals to a conventional loudspeaker 89. The output of intermediate frequency amplifier 52 is further coupled to a second channel comprising an intermediate frequency bandpass filter 96 having its output terminals connected to the input circuit of an amplitude modulation detector 97. The output terminals of detector 97 are connected to the input circuit of an audio amplifier 98 which applies signals to a conventional loudspeaker 99. The loudspeakers 89 and 99 are preferably spaced apart in a listening chamber or auditorium in the same manner as heretofore described in connection with the apparatus of FIG. 1.

Intermediate frequency amplifier 52 preferably has a sufficiently wide bandpass to provide reasonably linear amplification cf the amplitude modulation side bands and the frequency modulation side bands of the intermediate frequency carrier signal provided by the mixer circuit 51 when the receiver is tuned to receive an AM-F M multiplex signal of the general type transmitted by apparatus such as that of aforementioned application Serial No. 808,038. The intermediate frequency carrier has a center frequency i about which it is frequency modulated in accordance with the difference signal AB. The center frequency f may for example by 456 kilocycles las utilized in conventional AM receivers. In accordance with the present invention the bandpass filters or intermediate frequency selective networks 86 and 96 are provided with resonant tank circuits respectively comprising components 61 through 68 and 71 through 78. The tank circuit 61-68 is similar in circuit configuration to the oscillator tank circuit 11 to 18 of FIG. 1 with the corresponding components being similarly interconnected. Tank circuit 61-68 differs in that when switch arm 65 is in the normal position, that is contacting terminal 66 the tank circuit is resonant at a center frequency i which preferably is 456 kilocycles. With switch arm '75 in the normal position (contacting terminal 7 6) the second resonant circuit '7178 is likewise tuned to the same IF center frequency f With switch arms 65 and 75 so positioned converter 51 may be tuned by means of the conventional continuous tuning member 54 to select an RF. carrier signal and to produce a sound intermediate frequency carrier of frequency f in response thereto. Accurate adjustment of the converter tuning is achieved by the well known technique of adjusting for maximum sound volume from speakers 89 and 99. With converter 51 so adjusted both loudspeakers will produce balanced monophonic sound in concert in response to (l) ordinary monophonic AM signals; or (2) multiplex AM/FM stereo as described by application Serial No. 808,037; or (3) double-single sideband AM stereo signals as taught by the aforementioned January 1960 issue of Electronics.

To reproduce stereo intelligence from either of the latter two types of transmission it is only necessary to monophonically tune the receiver of FIG. 2 as described above, and then move switch arm 65 to terminal 67 and switch arm 75 to terminal 78. With the switches so readjusted the resonant frequency of tank circuit 616*8 is increased and the frequency of tank circuit 71-7 8 is correspondingly decreased. Accordingly, the lbandpass selective network 86 now has a frequency respouse characteristic as shown by curve in H6. 3. The geometrical center or maximum response portion of the bandpass characteristic 80 lies approximately 3.0-4.0 kc. above the center frequency f of the intermediate frequency carrier. For example, the center of the maximum response portion may be approximately 460' kilocycles so that the center frequency f lies on the positively sloping portion of the characteristic curve 80.

Similarly, bandpass selective network 96 now has a bandpass characteristic substantially as shown by curve of FIG. 3. The bandpass characteristic 90 has a negatively sloping portion which is substantially linear between the limits of frequency modulation i and f as shown in FIG. 3. It will be appreciated that the bandpass filter 9 6 may comprise any resonant circuit or amplifier or ordinary intermediate frequency transformer which is tuned or aligned to provide maximum response at the center frequency f when switch 75 is positioned at contact 76.

Operation of the receiver system of FIG. 2, when adjusted for stereo reception, will be best understood by particular reference to the curves of FIG. 3. In the center portion of FIG. 3 is shown a sine wave 69 which is frequency modulated and is constant inamplitude. While the actual signal output from the converter 51 of the apparatus of FIG. 2 is substantially always amplitude modulated as well as being frequency modulated, it is convenient for the purpose of analysis of the system to consider the constant amplitude sine wave 69 as shown in FIG. 3. Such a constant amplitude frequency modulated signal would occur, for example, when audio signals A and B are equal in amplitude and opposite in phase so that A+B equals zero and AB equals 2A. Curve '79 of FIG. 4 indicates the manner in which the frequency of the sine wave 69 varies in frequency as a function of time. At a time t the sine wave 69 has a frequency f for example 456' kilocycles. This condition corresponds to a time at which the stereophonic difference signal AB has a Zero value. At a later time t the frequency of wave 6-9 has increased to a maximum corresponding to the frequency f This condi-. tion indicates a maximum amplitude value of the stereophonic difference signal A -B. At a still later time, t the frequency of Wave 69 has decreased, through the mean frequency f to a minimum frequency value f as indicated by the minimum of curve 79. This con dition corresponds to a maximum negative value of the stereophonic difference signal AB.

Projection of the curve 79 to the sloping portion of the adjusted bandpass characteristic 80 of the IF bandpass filter 86 illustrates that as the intermediate frequency sine wave 69 decreases in frequency between the times t and i it moves down on the sloping portion of the bandpass characteristic 80 and is thereby progressively more attenuated. Thus, bandpass filter 86 modifies the amplitude of the intermediate frequency carrier Wave as a direct function of the frequency modulation thereof. The amplitude modulation component generated by the filter 86 is illustrated by curve 82 in FIG. 3. Curve 82 further indicates the manner in which the output of amplitude modulation detector 87 would vary if a constant amplitude frequency modulated carrier Wave such as that shown at 69 were applied to the input circuit of bandpass filter 86. When the frequency modulation of the sine wave 69 is considered as representing the stereophonic difference signal AB, then it is seen that the curve 32 at the output of detector 87 represents the audio frequency component resulting from slope demodulation of the stereophonic difference signal. If it be assumed that the frequency modulated wave 69 carries no amplitude modulation, then the audio output wave 82 would vary as a direct function of the stereophonic difference signal.

Considering the projection of curve 79 to the negatively sloping portion of the adjusted frequency response characteristic 90 of the bandpass filter 96, it may be readily seen that as the IF carrier frequency decreases between the times t and t the attenuation provided by filter 96 progressively decreases. Thus, the amplitude of the intermediate frequency carrier transmitted by filter 96 will progressively increase between the times t and t as shown by the curve 92 in FIG. 3. The amplitude modulation component thus generated by the filter 96 is detected by the amplitude modulation detector 97 and is applied through the audio amplifier 98 to the loudspeaker 99. The audio signal component resulting from the slope conversion action of filter 96 on frequency modulated wave 69 is illustrated by curve 92. It is important to note that curve 92 corresponds inversely to curve 82; thus, if the curve '82 represents a positive or direct function of the stereophonic diftference signal A'-B, then curve 92 represents a negative or inverse function of the stereophonic difference signal.

When the receiver of FIG. 2 is tuned to a signal of the type radiated by apparatus such as that of copendin-g application Serial No. 808,038, the intermediate frequency carrier as translated by intermediate frequency amplifier :52 will carry amplitude modulation corresponding to the sum signal A-+B. Filter 86 adds to the pre-existing A+B amplitude modulation, a second amplitude modulation component corresponding directly to the stercophonic difference signal AB. Accordingly, the modified amplitude modulation envelope as applied to \detector 87 varies substantially in accordance with the algebraic sum of the sum signal (A +3) and the stereophonic difference signal (AB). Thus, the audio output signal from amplitude modulation detector 87 will be representative of the left hand audio signal A.

Conversely, the filter 96 creates an amplitude modulation component which varies as an inverse function of the stereophonic difference signal AB. The amplitude modulation created by filter 96 adds to the pre-existing sum signal amplitude modulation as translated by IF amplifier 52, and the envelope amplitude of the intermediate frequency carrier at the output of filter 96 varies in accordance with the algebraic difference of the sum signal A+B and the stereophonic dilference signal AB. Thus, the envelope amplitude of the carrier signal as applied to the detector 97 varies substantially as a function of the audio frequency signal B from the right hand or second microphone of the transmitting studio.

While the present invention has been described and illustrated as utilizing manually operable switches in conjunction with a plurality of detuning capacitances it is not intended to be so limited. It will be apparent that the same side tuning results may be obtained by using switch means in conjunction with a plurality of taps on the oscillator coils in the apparatus of FIG. 1. Similarly, step detuning of the IF bandpass, as in 'FIG. 2, can readily be achieved by the use of tapped inductances or by means of voltage responsive variable capacitance members and means for supplying a direct current control potential having first and second predetermined voltage levels.

It will be apparent that the apparatus of FIG. 2 is compatible for reception of either monophonic or stereophonic signals transmitted in the AM broadcast band. That is, with switches 65 and 75 positioned to respectively contact terminals 66 and 76 the receiver of FIG. 2 may be tuned to any conventional AM broadcast station and will receive conventional monophonic AM signals to produce conventional monophonic sound in concert through the speakers 89 and 99 with the produced sound being in phase and mutually compatible.

While the present invention has been shown and described in certain preferred embodiments only, it will be understood by persons skilled in the art that it is not so limited, but is susceptible of various changes and modifications within the spirit and scope thereof.

We claim as our invention:

1. In a system for receiving a carrier wave having amplitude modulation and frequency modulation components respectively representative of a first signal A+B and a second signal AB, wherein A and B are signal components respectively corresponding to sound intelligence at first and second locations, first and second heterodyne converters responsive to said carrier wave for respectively producing first and second intermediate frequency carrier signals, each of said converters including a resonant tank circuit having an inductive reactance and a capacitive reactance connected in parallel, first and second intermediate frequency amplifiers coupled respectively to said first and second converters and resonant at a predetermined intermediate frequency, with the frequency response characteristic of said first amplifier having a positively sloping portion extending over a range of frequencies at one side of said predetermined intermediate frequency and the frequency response characteristic of said second amplifier having a negatively sloping portion extending over a range of frequencies at the other side of said predetermined intermediate frequency, first and second tuning means coupled respectively to the tank ciruits of said first and second converters for respectively tuning said first and second IF carrier signals to the maximum response frequencies of said first and second amplifiers; first and second switching devices connected respectively to the tank circuits of said first and second converters for shifting the resonant frequencies of said tank circuits by a predetermined amount, with each of said switching devices being operable to change the effective Value of one of said reactances for shifting the resonant frequency of the associated tank circuit by a predetermined amount to respectively reduce the first I'F carrier frequency and increase the second IF carrier frequency thereby placing said first and second IF carrier signal respectively on said positively sloping and negatively sloping portions of said frequency response characteristics.

2. Apparatus for receiving :a single carrier wave amplitude modulated with a first A+B and frequency modulated with a second signal AB wherein A and B are stereophonic signals representative of sound intelligence at first and second spaced locations, said receiver apparatus comprising an input signal path including a heterodyne frequency converter for producing IF carrier signals having the same amplitude and frequency modulations, said converter including tunable oscillatory circuit means for varying the frequency of said IF carrier signals over a predetermined frequency range; first channel means coupled to the output of said signal path and including in cascade connection a first resonant circuit normally having its maximum response at a predetermined intermediate frequency within said range, a first amplitude modulation detector and a first sound reproducer for reproducing sounds at intensity levels dependent upon the proximity of the IF carrier signal frequency to said predetermined intermediate frequency; second channel means coupled to the output of said signal path and including in cascade connection a second resonant circuit normally having its maximum response at said predetermined intermediate frequency, a second amplitude modulation detector and a second sound reproducer for producing sound at intensity levels dependent upon said proximity; said first and second resonant circuits having frequency response characteristic curves which respectively include a positively sloping portion at frequencies below the resonant frequency thereof and a negatively sloping portion at frequencies above the resonant frequency; tuning means coupled to the oscillatory circuit means of said converter for adjusting said IF carrier signal approximately to said predetermined intermediate frequency with said adjustment being indicated by maximum intensity sound reproduction from both said channels; first and second switching devices connected respectively to said first and second resonant circuits and respectively operable to change the efiective value of an impedance therein for oppositely shifting said frequency response characteristics to place said IF carrier signal intermediate said positively sloping portion of the response curve of said first resonant circuit and intermediate said negatively sloping portion of the response curve of said second resonant circuit so that said first channel slope detects said frequency modulation and amplitude detects said amplitude modulation and additively combines the detection products to reproduce said s'tereophonic signal A, and so that said second channel inversely slope detects said frequency modulation and amplitude detects said amplitude modulation to reproduce said stereophonic signal B.

12 References ited in the file of this patent UNITED STATES PATENTS 2,061,818 Weyers Nov. 24, 1936 2,164,082 Day June27, 1939 2,261,628 Lovell Nov. 4, 1941 2,437,910 Crosby Mar. 16, 1948 OTHER REFERENCES Wireless World, Binaural Transmission, May 1941, pp. 1130431. (Copy in Div. 51.)

Article by Sweeney, Electronics, vol. 32, No. 19, May 8, 1959 (pp. 56-58). 

1. IN A SYSTEM FOR RECEIVING A CARRIER WAVE HAVING AMPLITUDE MODULATION AND FREQUENCY MODULATION COMPONENTS RESPECTIVELY REPRESENTATIVE OF A FIRST SIGNAL A+B AND A SECOND SIGNAL A-B, WHEREIN A AND B ARE SIGNAL COMPONENTS RESPECTIVELY CORRESPONDING TO SOUND INTELLIGENCE AT FIRST AND SECOND LOCATIONS, FIRST AND SECOND HETERODYNE CONVERTERS RESPONSIVE TO SAID CARRIER WAVE FOR RESPECTIVELY PRODUCING FIRST AND SECOND INTERMEDIATE FREQUENCY CARRIER SIGNALS, EACH OF SAID CONVERTERS INCLUDING A RESONANT TANK CIRCUIT HAVING AN INDUCTIVE REACTANCE AND A CAPACITIVE REACTANCE CONNECTED IN PARALLEL, FIRST AND SECOND INTERMEDIATE FREQUENCY AMPLIFIERS COUPLED RESPECTIVELY TO SAID FIRST AND SECOND CONVERTERS AND RESONANT AT A PREDETERMINED INTERMEDIATE FREQUENCY, WITH THE FREQUENCY RESPONSE CHARACTERISTIC OF SAID FIRST AMPLIFIER HAVING A POSITIVELY SLOPING PORTION EXTENDING OVER A RANGE OF FREQUENCIES AT ONE SIDE OF SAID PREDETERMINED INTERMEDIATE FREQUENCY AND THE FREQUENCY RESPONSE CHARACTERISTIC OF SAID SECOND AMPLIFIER HAVING A NEGATIVELY SLOPING PORTION EXTENDING OVER A RANGE OF FREQUENCIES AT THE OTHER SIDE OF SAID PREDETERMINED INTERMEDIATE FREQUENCY, FIRST AND SECOND TUNING MEANS COUPLED RESPECTIVELY TO THE TANK CIRUITS OF SAID FIRST AND SECOND CONVERTERS FOR RESPECTIVELY TUNING SAID FIRST AND SECOND IF CARRIER SIGNALS TO THE MAXIMUM RESPONSE FREQUENCIES OF SAID FIRST AND SECOND AMPLIFIERS; FIRST AND SECOND SWITCHING DEVICES CONNECTED RESPECTIVELY TO THE TANK CIRCUITS OF SAID FIRST AND SECOND CONVERTERS FOR SHIFTING THE RESONANT FREQUENCIES OF SAID TANK CIRCUITS BY A PREDETERMINED AMOUNT, WITH EACH OF SAID SWITCHING DEVICES BEING OPERABLE TO CHANGE THE EFFECTIVE VALUE OF ONE OF SAID REACTANCES FOR SHIFTING THE REASONANT FREQUENCY OF THE ASSOCIATED TANK CIRCUIT BY A PREDETERMINED AMOUNT TO RESPECTIVELY REDUCE THE FIRST IF CARRIER FREQUENCY AND INCREASE THE SECOND IF CARRIER FREQUENCY THEREBY PLACING SAID FIRST AND SECOND IF CARRIER SIGNAL RESPECTIVELY ON SAID POSITIVELY SLOPING AND NEGATIVELY SLOPING PORTIONS OF SAID FREQUENCY RESPONSE CHARACTERISTICS. 